Automatic merge from submit-queue
kubectl: deploy generators don't need to impl Generator iface
The `kubectl create deployment` generators do not need to implement the Generator interface, since they are only used as implementations of the StructuredGenerator interface. I was able to delete some tests of their Generator methods as part of this change.
### Considerations for code reviewers
1. Every other StructuredGenerator implementation implements the Generator interface in additional. My change makes the "create deployment" generators a little unusual. I've added a docstring to this effect in `util/factory_client_access.go`.
2. This significantly reduces the maintenance / testing burden for future updates to `kubectl create deployment`.
**Release note**:
```release-note
NONE
```
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